In a healthy human heart, an increase in heart rate results in an increase in force of contractions (positive force-frequency relationship, or FFR), and an acceleration of contractile kinetics (frequency-dependent acceleration of relaxation, or FDAR). In human heart failure, the FFR flattens or even becomes negative, while FDAR greatly diminishes; these two phenomena are classic hallmarks of heart failure. Despite its critical role in health and disease, frequency- dependent modulation of contraction and kinetics is poorly understood. The magnitude in contractile response to frequency response in mice is >10 times smaller than in humans, and most often even completely absent. This necessitates the investigation of frequency-dependent processes of contraction and kinetics in a large animal model, or better yet, in human myocardium. Guided by the literature, our own data, and the last 5 years of preliminary data from experiments on human heart tissue and its frequency-dependent regulation, we recently wrote a very extensive review that postulates the hypothesis that the kinetics of relaxation are governed by three interdependent processes: intracellular calcium decline, myofilament calcium binding kinetics, and cross-bridge cycling kinetics. Currently, there is a vast vacuum of data regarding the kinetic rate and regulation in human myocardium, which is critical, since data from the 10-times faster murine myocardium are nearly impossible to interpret in the context of the prevailing rates in the human. We have constructed aims that 1) first establish the kinetic rates for the three processes that govern relaxation in failing and non-failing human myocardium, and then proceed to 2) modify this kinetic rate by engineering a different myofilament calcium sensitivity in human failing and non-failing myocardium. In the first aim, we will assess the kinetic rates of calcium transient decline, myofilament responsiveness, and cross-bridge cycling kinetics in non-failing and failing human myocardium, in the second aim, we will assess whether modification of the kinetic rate governing myofilament calcium responsiveness can restore the FDAR in failing human myocardium by employing engineered TroponinC proteins. The above aims will be carried out in n>50 failing and n>50 non-failing human hearts. This will allow us to eventually perform a differential analysis based on disease etiology. Based on our experience in working with human hearts we will have sufficient statistical power to distinguish outcomes between hearts classified as ischemic-cardiomyopathy, dilated-cardiomyopathy, and those with a primary diastolic dysfunction.